Field of the Invention
The present invention relates to a new and improved method for dehydrating water-miscible organic liquids and, more particularly, relates to a new and improved method for dehydrating glycols such as diethylene glycol, triethylene glycol, tetraethylene glycol, and mixtures thereof.
The energy crisis, the gas shortage and the need to compress declining gas reserves to pipeline pressures has brought technical changes to the gas industry along with a higher price for natural gas. Compressing gas to higher pressure levels, with the attendant temperature rise, requires dehydration facilities capable of a larger dewpoint depression than has been necessary before. Where triethylene glycol is used to dehydrate gas, now it must be concentrated to levels substantially above the usual 98.5-99.0% by weight available from atmospheric distillation of glycol-water mixtures at temperatures up to about 390.degree. F.
Since the early days of gas dehydration with glycols, gas stripping has been used to remove water traces from triethylene glycol solutions. In recent years an improved gas stripping method similar to that disclosed in U.S. Pat. No. 3,105,748 has been used to concentrate triethylene glycol to very high levels (99.2-99.9 plus percent by weight) using moderate quantities of stripping gas. However, as reserves have dwindled and the price of gas has increased, operators are taking a critical look at gas consumption in all gas-consuming facilities in a sincere effort to make more economical, effective use of it in field operations. Dehydrators using gas-stripped triethylene glycol can be expensive to operate and alternative means for stripping water traces from glycol have been sought.
Concerning the natural gas industry which is faced with tremendous demand, declining reserves and controlled prices, there is need for thermally-efficient, lightweight compact processes to properly dehydrate hot, compressed gas to pipeline specifications at locations either on-shore or on off-shore platforms. Particularly off-shore, weight, deck space and floor loading is as important as performance capability and contemporary dehydration processes leave much to be desired in these respects in addition to being thermally inefficient.
Some of the methods used, besides gas stripping, are vacuum distillation, Polderman, L. D., The Performance of Vacuum-regenerated Glycol-type Natural Gas Dehydration Plants, Proceedings of the University of Oklahoma Gas Conditioning Conference, 1957; and azeotropic distillation with toluene, iso-octane, and the like, known as the DRIZO Process, Fowler, A., "Super-Drizo"--The Dow Dehydration Process, Proceedings of the University of Oklahoma Gas Conditioning Conference, 1975; Pearce, R. L. and Protz, Drizo--Improved Regeneration of Glycol Solution, Proceedings of the University of Oklahoma Gas Conditioning Conference, 1972. Still another method is disclosed in my prior U.S. Pat. No. 3,589,984 known by the name COLDFINGER.RTM.. It is a method for exhausting water traces from organic liquids which are miscible with water.
By reference to the vapor-liquid diagram for triethylene glycol at any pressure, for any liquid concentration, the corresponding equilibrium vapor is substantially more rich in water. Then consider a closed tank, half full of 99% by weight triethylene glycol at its boiling point and note that equilibrium vapor, at atmospheric pressure, is approximately 35% triethylene glycol and 65% water. Now insert a "cold finger" or condenser in that vapor space to cause condensation. The condensate drops off the cool surface and, if equivalent heat were applied to balance the heat loss from the apparatus, equilibrium would be re-established.
But if a trough were placed under the "cold finger" to collect the condensate and drain it from the system, the equilibrium would be upset and the liquid phase would further vaporize to approach equilibrium again, removing additional water from the liquid. In time the water in the liquid phase would be exhausted and the residual liquid would approach 100% concentration.
This is the essential principle of the water exhauster device as used in the method of the present invention to remove trace quantities of water from a glycol-water solution. The method is close to fractionation, evaporation and distillation drying, and yet it is uniquely different from each.
This water exhauster has numerous variations which can be compounded. For example, the system can be operated at near-atmospheric pressure or at a pressure (vacuum) as low as about 200 mm. Hg. Heat may be finished by the feed stream or it may come from extraneous sources. Coolant for the condenser tube bundle may be the glycol withdrawn from the adsorber en route to be reconcentrated, or it can be a separate extraneous coolant. The coolant can operate at absorber temperature or at any other, preferably lower, temperature. The hot vapor from the water exhauster can circulate by convection or it can be moved past the condenser by a blower or fan. The liquid can be batched or flowed by gravity through a controlled labyrinth from inlet to outlet and it can be agitated mechanically.